Glossary

What Is an SPI A1 Finish? Injection Molding Explained

SPI A1 is mirror polish on the cavity side for lens-grade land, not a default gloss for whole enclosures.

Published October 15, 2026

Engineering note

SPI A1 is mirror grade mold polish, not a material specification

Reserve A1 for faces that must read as near mirror after molding and accept the tooling and process cost.

A1 uses sequential grade A diamond polish on cavity steel to reach the top band on the SPI chart.

Shops often describe the last mirror passes as grade 3 diamond buff with a fine grit sequence such as 6000 grit before final buff.

SPI A1 labels mold cavity polish. It is not the same as an optical surface specification for imaging lenses.

The plastic surface follows steel quality, resin clarity, gate location, venting, and pack profile. Mirror intent still needs process control.

Quote A1 with per face maps, cosmetic acceptance rules, and gate plans before final polish hours are fixed.

Reference

SPI A1 vs nearby grades (typical mold steel targets)

Ra ranges describe steel prep shorthand, not guaranteed part Ra. The same A1 steel can look different by resin, color, and gate location. Mass production acceptance should use signed T0 parts or a finish plaque, not Ra alone.

Finish	Method	Gloss	Ra (approx.)	Typical use
SPI A1	Grade A diamond polish (grade 3 buff, ~6000 grit sequence)	Mirror gloss	0.012 to 0.025 um	Lenses, mirror caps, highest cosmetic land
SPI A2	Fine diamond polish	High gloss	0.025 to 0.05 um	Visible housings without full mirror scope
SPI A3	Fine abrasive polish	Medium gloss	0.05 to 0.10 um	Consumer electronics with moderated glare

Selection

When SPI A1 is a strong choice

A1 pays off when reflection quality is part of the product definition.

Optical windows, lens bezels, or branding faces that must read mirror under inspection lighting
Small Class A land areas where polish labor is limited to hero surfaces
Programs with defined gate hide strategies and tight cosmetic acceptance on gloss faces
Parts in clear or high gloss colors where steel defects transfer visibly

Selection

When to avoid SPI A1 or step down a band

A2 or B1 often covers visible housings at lower polish risk.

Default to A1 on entire enclosure

Large flat faces that amplify flow marks, sink, and gate blush
High annual volume tools where mirror polish repair slows maintenance
Assemblies where only a small badge needs mirror and the rest is matte
Drawings with no gate or parting line cosmetic rules on gloss zones

Scope A1 deliberately

Limit A1 to mapped faces with shutoffs to A2, B, or textured zones
Move large flats to A2 or B1 when reflection spec allows
Plan EDM and weld removal before polish so recast does not chase A1 rework

DFM

How A1 changes mold and process risk

Mirror steel exposes any fill imbalance or steel defect.

Gate blush, weld lines, sink, and hesitation marks show immediately on A1. Gate style, location, and valve timing need early review.
EDM recast and weld repairs must be removed and re polished or gloss will not recover locally.
Soft or wrong cavity steel for diamond polish can pit or haze. Hardened or corrosion resistant tooling suited to mirror polish belongs in the mold spec before A1 hours are quoted.
Mixed A1 with texture requires steel shutoffs and clear parting line witness limits.
Ejector pins, slide marks, and parting line flash on A1 land need design hide features or written acceptance rules.

EDM glossary entry Stainless mold tooling glossary

3D CAD, material, quantity, tolerance band, and insert notes for an instant injection molding quote

Materials

Resin and color effects on A1 appearance

Not every family reads mirror on A1 steel. Match material to the cosmetic target.

Clear PC, PMMA, and high gloss ABS often suit small mirror land when grade, drying, and gate plan are controlled.
Unfilled PP, soft TPU, and glass filled PA on large A1 faces often look hazy or show fiber or flow on mirror zones.
Clear PC or acrylic programs need resin clarity and drying discipline, not only steel polish.
Glass filled grades rarely achieve true mirror on large faces even when steel is A1.
Dark high gloss colors show sink and flow more than matte textures on the same geometry.
Lock color and grade for cosmetic builds. Regrind shifts can change gloss batch to batch.

Drawings

How to call out SPI A1 on CAD and PDF

Separate mirror land from the rest of the part before mold build starts.

Mark A1 faces on CAD or PMI, including allowed parting line and gate witness zones
State cosmetic reject limits for flow, sink, scratch, and dig on A1 surfaces for T1
Reference SPI chart or customer finish standard. Cross reference optical polish if SPI alone is insufficient
Note gate type and location constraints relative to A1 land
Attach prior part or plaque reference when mirroring an existing cosmetic standard
State that cosmetic signoff uses physical samples or a finish plaque, not Ra numbers alone on the drawing

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